1. Technical Field
The present invention relates to a method and apparatus for measuring isometric and concentric strength and muscular endurance of a human subject's extensor and flexor muscles used in rotating the trunk of the subject.
2. Descripticn Of The Prior Art
The generally accepted measurement criterion for the maximum tension which can be exerted by a muscle is the maximum moment of force that a muscle can exert on a body part. This is referred to as the maximum strength of the muscle and is expressed, for example, in kilograms per square centimeter of transverse section. In order to be meaningful, this measurement parameter must be expressed relative to the length of the muscle or the position of the body part. Several factors influence the validity of muscle strength tests. First, the muscle or muscle groups to be tested must be isolated from muscles which may contribute a moment to the measurement apparatus. This is accomplished by positioning the subject appropriately and stabilizing the parts of the body containing muscles which are not part of the measurement. Second, the moment or torque measuring device must be oriented perpendicular to the moment arm of the body part. The measuring device must also operate in the plane of motion of the body part. This requires a reasonable estimate of the axis of rotation of the body part involved.
Strength measurements should be taken at a proportional distance from the axis of rotation of the body part comparing measurements from different subjects. This is particularly important when the force exerted by the body part is the parameter to be measured. With respect to this methodology, moment measurements should also be taken at a proportional distance from the axis of rotation.
A widely used device for measuring strength and endurance of human muscles is the Cybex II, manufactured by Cybex, a division of Lumex, Inc., of Bay Shore, N.Y. This device is described in various catalogues produced by the manufacturer and also in U.S. Pat. No. 3,465,592 to Perrine, which patent is expressly incorporated herein by reference. In this apparatus, muscular strength and endurance is measured as a function of the torque or moment applied by muscular exertion to rotate an input shaft about its axis. The Cybex II is a passive component and does not apply force to the subject; it merely measures the torque applied to the device by the subject. A speed control unit permits the maximum rate of angular displacement of the input shaft to be set from 0.degree. per second to 210.degree. per second. When the speed control is set at 0.degree. per second, the subject is able to apply torque to the input shaft but the input shaft does not rotate. This configuration is used to measure isometric strength of the muscle involved. When the speed control is set at values greater than 0.degree. per second, the input shaft will rotate, in response to applied torque from the subject, through a range of motion at a constant velocity for as long as the subject applies torque to the system. This latter configuration is employed to measure the concentric strength and muscular endurance of the subject.
Although the Cybex II is useful in exercising trunk flexion and extension, it does not properly measure the strength and endurance of the flexors and extensors of the lumbar and thoracic spine. The reason for this is that the Cybex II does not provide two (2) critically important features which are required to accurately measure the strength and endurance of the flexors and extensors. Specifically, the Cybex II does not provide for immobilization of other muscles which a subject involuntarily employs in flexing and extending his or her trunk about the axis of rotation of spinal flexion and extension. Further, the Cybex II provides no means for accurately positioning the axis of rotation of spinal flexion and extension with respect to the axis of the torque transducer input shaft; that is, these two (2) axes must be reasonably aligned in order to have valid strength and endurance measurements.